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Passengers on a United Airlines plane that was diverted after a threatening note was discovered on board had to offer samples of their handwriting to the police investigating who had written it.

Flight UA971 from Rome to Chicago was forced to make an emergency landing at Shannon Airport in Ireland on Monday local time after the note, which reportedly made reference to a bomb on the plane, was found in the bathroom.

The note was apparently scrawled onto the surface of one of the toilets, the Irish Times reported.

Flight UA971 from Rome to Chicago had to make an emergency landing in Ireland after the note was discovered. Photo / Flightradar24

Irish police then began the huge task of investigating the incident, which included taking handwriting samples from passengers so they could determine the author, according to the Irish Times.

Passengers were searched and had their hand baggage removed and screened. Luggage in the cargo hold was also examined.

The United States government is also investigating the incident. A US government source told Reuters similar incidents involving threatening notes found on planes happened about once a week.

United Airlines Boeing 767-322 #UA971 flight from Rome to Chicago was diverted to Shannon Airport in Ireland on Monday after a hand writing message referred to a bomb on the plane was discovered on board pic.twitter.com/VmmuUCpxDV